The performance of a ship falls off either when marine organisms grow on the sub-marine parts of her hull, or when the topcoat paint is applied irregularly or becomes rough during service.
The extent of marine fouling is controlled by applying to the hull a top coat paint comprising anti-fouling agents. The anti-fouling agents are biocides which are freed from the paint surface at a rate such that their concentration is lethal to marine organisms near the surface.
Anti-fouling paint compositions containing organo-metallic poisons which are chemically bound therein have been previously proposed.
British Pat. No. 1,124,297 discloses that a considerable increase in the anti-fouling efficacy of such paints is achieved by using pigments which have an appreciable water-solubility when incorporated in large quantity by comparison with the quantity of paint vehicle, and particularly if the pigments have a toxicity to marine organisms in their own right. However, the surface of the coating becomes progressively rougher due to the dissolution of part of the composition.
British Pat. No. 1,457,590, which is hereby incorporated by reference in its entirety, discloses a film-forming copolymer containing 50-80 wt.% triorganotin monomer, which gradually dissolves so that (i) a fresh anti-fouling paint surface is continuously revealed, and (ii) moving seawater smooths the surface of the coating. For environmental and other reasons, the triorganotin content is considered too high.
European patent application No. 51,930 (U.S. Pat. No. 4,407,997) discloses certain polymers of acrylate esters which can be used as the basis of erodible marine paints. The marine paints disclosed comprise a film-forming acrylic polymer and a sparingly soluble metalliferous pigment. The acrylic polymer comprises (A) 20-100 wt.% of monomer units selected from methyl acrylate and ethyl acrylate; (B) 0-45 wt.% of monomer units of a triorganotin salt of an olefinically unsaturated carboxylic acid; and (C) 0-80 wt.% of monomer units selected from C.sub.1 -C.sub.4 alkyl methacrylate and styrene. Monomer units (A), (B) and (C) form at least 80 wt.% and the balance, if any, consists of monomer units of at least one other olefinically unsaturated comonomer. However, marine paints according to No. EP-51,930 have the disadvantage of showing considerable blistering and flaking. Due to the low hydrolysis rate of this copolymer, the self-polishing effect is not fully satisfactory.
Belgian Pat. No. 901,426 discloses marine paints comprising a film-forming polymer made from up to 60 wt.% of triorganotin acrylate or methacrylate; 1-80 wt.% of C.sub.1 -C.sub.2 alkoxy C.sub.1 -C.sub.2 alkyl(meth)acrylate; and 20-85 wt.% of C.sub.1 -C.sub.4 alkyl methacrylate. However, the coatings obtained are relatively soft and therefore are more sensitive to mechanical damages. Moreover, some of the hereabove compositions contains TBTMA (triorganotin methacrylate) and high amounts of alkyl methacrylate do not show a sufficient hydrolysis rate, thus leading to the drawbacks well known in the art, particularly about the anti-fouling properties.
Many countries are introducing new stringent water quality programs, some of which call for a phased reduction in the triorganotin content of copolymer paints. The lowest efficient limit has been determined by T. A. Banfield (J. Oil Col. Chem. Assoc. 63, 93, 1980) to be 1 ug tributyltin oxide/cm.sup.2.day. The lowest theoretical triorganotin monomer content in the binder may therefore be evaluated at about 10 wt.% for an erosion rate of 5 um/month.
There is therefore a need in the art for improved erodible anti-fouling paint compositions having a reduced content of organotin monomer while retaining the good anti-fouling and self-polishing properties of the anti-fouling paints of the prior art which contain 50-80 wt.% of organotin monomer.